1. Field of the Invention
The present invention relates to the field of digital cellular wireless telecommunications. More particularly, the invention relates to the use of pilot beacons to facilitate call handoffs in digital CDMA, wireless telecommunications networks.
2. Description of the Prior Art
Conventional cellular phone systems divide a city or other service area into a number of cells that are each equipped with a transceiver. As a cellular telephone moves from one cell to another, it is “handed off” from cell to cell by a mobile switching center, which determines from which cell the signal strength is strongest.
There are two basic forms of call handoffs: hard handoffs and soft handoffs. Hard handoffs are performed on a “break before make” basis, requiring a call connection to be broken in the original cell before it is made in the next cell. Hard handoffs are required in systems that use frequency division multiple access (FDMA) and time division multiple access (TDMA) because these systems employ different frequencies in adjacent cells and therefore require a call to be transferred from one frequency to another when transferring between cells.
In contrast, soft handoffs employ a “make before break” routine where a call is connected to a new cell before it is broken from the previous cell. Soft handoffs can be provided by wireless communication services using code division multiple access (CDMA) techniques. CDMA is a digital spread-spectrum modulation technique that digitizes wireless conversations and tags them with special codes. The digitized data is spread across the frequency band in a pseudo random pattern. Receiving mobile phones are instructed to decipher only the data corresponding to particular codes to reconstruct the signal. CDMA networks provide soft handoffs because they do not require the use of different frequencies in adjacent cells. However, as described below, even CDMA networks require hard handoffs in certain circumstances where an increase in capacity is warranted above what is provided by one RF frequency.
A typical CDMA PCS telecommunications system is comprised of one tier of cells that operates at one frequency F1. Each cell includes a base station unit and three sector antennas that together provide wireless communications in three sectors of the cell each covering 120°. One method to increase the capacity of this system is to add a second tier of cells on the top of the first tier at another frequency F2. Usually the first tier at F1 has wider area coverage in comparison to the second tier at F2. A problem arises when a mobile is moving from the F2 coverage area to an area with only F1 coverage. This would require a hard handoff. Sometimes the hard handoff from F2 down to F1 does not occur smoothly and might result in a dropped call.
One prior art solution to the above problem has been to add at the cell adjacent to the edge coverage of F2 a base station that supports a pilot beacon at F2. The pilot beacon base station has only three channels: a pilot channel, a paging channel, and a synchronization channel. This is not an economical solution as it results in the addition of a nearly complete second base station to act as the pilot beacon to assist in the hard handoff process. However, the current IS-95 A/B uses a predefined set of sixty-four CDMA Walsh codes for each sector, with one of the codes used for pilot channel, another for synchronization, one commonly used for paging, and the remainder available for ordinary traffic. In total one would expect to use more than 32 codes. The rest will be used for the pilot beacon.
Thus, using two or more frequencies in CDMA telecommunications cells requires hard call handoffs when a call on a first frequency must be handed off to an adjacent cell at a second frequency. Adding more base stations to add more traffic channels therefore eliminates some of the call handoff advantages of CDMA systems. The use of multiple base stations or pilot beacons in a cell is also an expensive solution because it requires additional equipment.